Historically, extra-dimensional theories of gravity were introduced soon after Einstein's discovery of the general theory of relativity. In the original form
extra dimensions were introduced by Kaluza and Klein (KK) to unify electromagnetism with gravity, the only two fundamental forces known at the time [1]}, [2]}. Extra-dimensional models have continued to evolve since the late 1970's, thanks in large part to the development of string theory. Over the last three decades, low-energy realizations of extra-dimensional models gained prevalence as well-motivated scenarios of physics beyond the standard model. One of the most popular and phenomenologically-viable models of extra dimensions is the Randall-Sundrum model [3]}, [4]}, wherein a compact extra dimension in Anti-de Sitter space is used to generate relative exponential factors; this factor allows particles fixed to a brane to interact at electroweak strength while also ensuring bulk-propagating gravity is weak in the observed extended four-dimensional space, thus providing a geometric solution for the hierarchy problem of the standard model.
